This invention relates to a method and apparatus to indicate tension imposed on a cable by a pull line during a cable placement operation. More particularly, the present invention provides for the measurement of tension generally at the point of attachment of a pull line to a cable comprised of, for example, optical fiber and for the transmission of a signal corresponding to the measured tension only on the cable to a site remote to the course of travel by the cable during the cable placement operation.
Tension meters, such as roller-type, spring-displacement type, or any other tension meter used to measure tension externally of a conduit or duct, reveal tension in the pull line. The measurement of tension in this manner cannot be relied upon to accurately indicate the tension applied to the cable by the pull line in a conduit or duct. A resistance against pull line movement in a duct can exceed 500 pounds, for example, without tension being applied to the cable. The conduit or duct walls, particularly at bends, restrain the pull line when it is straightened to advance the cable under tension. The tension on the pull line is not constant but changes from time-to-time disproportionally to the tension applied to the cable while the latter advances along the duct walls.
When the cable is comprised of optical fiber, measuring the tension on the pull line externally of the duct is not sufficiently accurate. When the pull line tension exceeds 500 pounds without any tension being applied to the cable, optical fibers of a cable are particularly susceptible to fatigue fractures due to an excursion of tension stresses beyond the yield point of the optical fiber material. Even though the tension on one or more fibers in an optical fiber bundle exceeds the yield point of the material, the loss of integrity of the fiber may not immediately occur. The fiber may fracture, thus losing its integrity, at a time long after tension has been relaxed and the cable is placed in service. Because of this phenomenon, tensioning of a cable comprised of optical fibers must be controlled with greater care during pulling in a conduit than a cable comprised of metal conductors. To facilitate the pulling operation of a cable, particularly optical fiber bundles, it is desirable, therefore, to accurately measure the magnitude of tension imposed on the cable itself and, in certain instances, to record measurements of tension throughout the pulling operation for future reference. This is true for cable made up of metal conductors since the conductors are subject to damage due to excessive tensioning during pulling in a duct. However, the phenomenon of fatigue fracturing in the absence of tension does not usually take place in metal conductors.
Sometimes a cable is placed for support aboveground by an overhead support line. Cable supports along the line impede movement of the cable along the support line which increases cable tension to varying degrees as the cable moves along the course aboveground. In aboveground cable placing operations, it is equally important to accurately determine the actual tension on the cable rather than a tension measurement affected by a resistance to movement by the pull line.